Polytrichastrum longisetum is a circumpolar boreo-temperate species, and is also found in the Southern Hemisphere. In northern Europe, the species occurs on well-drained acidic soil in heaths and moorlands, also in woods; the seta is often conspicuously longer than the leafy shoots (whence the name), and the capsules broadly ovoid, widest below the middle. As represented in eastern North America, P. longisetum is similar in habit and habitat to P. formosum var. densifolium and P. pallidisetum, but differs in the less strongly differentiated leaf sheath, short-rectangular sheath cells, and broad marginal lamina. Both P. formosum and P. longisetum have the marginal cells of the lamellae scarcely differentiated, in profile entire to finely serrulate by the projecting leading angles of the marginal cells; in P. pallidisetum the lamellae are crenulate in profile, and the marginal cells in cross-section broadened, flat-topped to shallowly retuse. Specimens identified as Polytrichum gracile var. anomalum, found in the wettest habitats, even submerged, are sometimes startlingly Atrichum-like in appearance. The leaf sheath is weakly developed and scarcely broader than the blade, the 1-stratose lamina is up to 20 cells wide, and the lamellae are few in number and confined to the median portion of the blade.